1. Field of the Invention
The present invention relates to a window glass antenna apparatus to be disposed on a window glass of an automobile for receiving radio broadcasts.
2. Description of the Related Art
In Japanese Utility Model Publication No. HEI 4-45312, there is disclosed a window glass antenna apparatus which comprises an antenna for receiving AM/FM radio signals, a signal cable, a first impedance transforming circuit connected between the signal cable and antenna for transforming the impedance in a frequency band of AM radio signal (AM frequency band) from high impedance to low impedance, a first filter circuit connected between the signal cable and antenna for allowing passage of FM radio signals, a second impedance transforming circuit connected between the signal cable and an antenna input circuit of a radio receiver for transforming the impedance in the AM frequency band from low impedance to high impedance, and a second filter circuit connected between the signal cable and antenna input circuit for allowing passage of the FM radio signals.
Each of the first and second filter circuits includes an LC series resonance circuit. An equivalent circuit for the AM frequency band is illustrated in FIG. 8 hereof for explaining the principle wherein the capacitance of filter is taken into consideration.
In the figure, Co collectively represents antenna effective capacitance existing between the antenna and ground, and antenna reactive capacitance existing at the antenna output in series therewith. An AM radio signal received by the antenna is represented as an AC power source Vi. Cb represents cable capacitance of the signal cable 6. Reference numerals 22 and 25 are transformers forming the first and second impedance conversion circuits.
However, the window glass antenna apparatus disclosed in Japanese Utility Model Publication No. HEI 4-45312 has a drawback in that due to capacitance component Cf of the LC series resonance circuits forming the first and second filter circuits, the antenna impedance decreases, thereby decreasing sensitivity for the reception of AM radio signals.